Asparagine signals mitochondrial respiration and can be targeted to impair tumour growth
Mitochondrial respiration is critical for cell proliferation. In addition to producing ATP via the electron transport chain (ETC), respiration is required for the generation of TCA cycle-derived biosynthetic precursors, such as aspartate, an essential substrate for nucleotide synthesis. Because mTORC1 coordinates availability of biosynthetic precursors with anabolic metabolism, including nucleotide synthesis, a link between respiration and mTORC1 is fitting. Here we show that in addition to depleting intracellular aspartate, ETC inhibition depletes aspartate-derived asparagine and impairs mTORC1 activity. Providing exogenous asparagine restores mTORC1 activity, nucleotide synthesis, and proliferation in the context of ETC inhibition without restoring intracellular aspartate in a panel of cancer cell lines. As a therapeutic strategy, the combination of ETC inhibitor metformin, which limits tumour asparagine synthesis, and either asparaginase or dietary asparagine restriction, which limit tumour asparagine consumption, effectively impairs tumour growth in several mouse models of cancer. Because environmental asparagine is sufficient to restore proliferation with respiration impairment, both in vitro and in vivo, our findings suggest that asparagine synthesis is a fundamental purpose of mitochondrial respiration. Moreover, the results suggest that asparagine signals active respiration to mTORC1 to communicate biosynthetic precursor sufficiency and promote anabolism.